1. Field of the Invention
This invention relates to accelerographs and particularly to those of the strong motion type which are employed to detect and record such movements of building structures as are caused by a range of earthquake shocks.
2. Description of the Prior Art
As set forth in an article entitled "MODERN TRENDS IN STRONG MOVEMENT (Strong Motion) INSTRUMENTATION" by H. T. Halverson, which appeared in "PROCEEDINGS OF CONFERENCE ON DYNAMIC WAVES IN CIVIL ENGINEERING" Swansea, Wales, 1970, John Wiley & Sons, the development of modern instrumentation for the detection and recording of strong local earthquakes commenced approximately in 1932, and has resulted in the design and availablility of a number of different types of devices based upon several different approaches to obtaining the desired measurements. These approaches have been in the form of (1) light reflection upon photogtaphic film; (2) magnetic tape; and (3) piezoelectric or piezo resistive devices. The present invention is directed to an improved accelerograph utilizing the first and second of the three listed approaches.
As also pointed out in the article referred to above, the need for improved accelerographs has become increasingly important since the building codes of certain major cities in the United States now require the installation of several of these devices in new building structures exceeding a certain number of floors.
While presently available accelerographs of the types described in the Halverson article have been reasonably effective in detecting and recording movements of certain intensities and direction, each has certain limitations. In particular, the light-reflecting types of accelerographs require the longest possible light paths in order to produce the most significant photographic shock pattern indications. However, where a small portable unit is to be constructed, in order to provide light paths as long as possible in a minimum amount of space, such paths must be folded a number of times. Efforts in this direction may be seen in the U.S. Patents issued to W. W. Miller (No. 2,670,660) and J. E. Jenkins (No. 2,917,918).
In addition, a problem with all types of accelerographs is providing an initiating switch which will activate the equipment instantly at the very first significant shock and keep it in operation for a long enough period to record such shocks and any further shocks which could reasonably be expected to follow it. Otherwise the equipment must be kept turned off so as not to waste power and the recording element, such as photographic film or magnetic tape. This requires a very fine control adjustment.
It has also been a problem, particularly with light-reflective accelerographs, to dampen the movement of the suspended mirrors once such movements have been initiated by any shock wave. Unless effective damping is accomplished, the indications of subsequent, closely following, smaller shock waves might be difficult to distinguish among the diminishing oscillations engendered by a preceding high intensity shock. Moreover, spurious undampened resonances may adversely affect trace width. Viscous dampening, however, presents certain difficult container and movement problems. While U.S. Pat. No. 3,333,460 issued to N. P. Reeff has suggested the use of electromagnetic damping, the mass-optical element and coil arrangement suggested by this patent, does not readily lend itself to utilization in small portable tri-axial accelerographs.
Any accelerograph should also be entirely self-contained and capable of withstanding very destructive forces. Thus, if it is light-reflecting, it should provide its own light source. In any event, it should be so encased that it can withstand the forces which might be exerted upon it by the collapse of the building in which the accelerograph is installed as well as resist any encroachment by water which might result from the rupture of pipes during an earthquake.
One of the great needs in an earthquake detection and damage control is to be able to utilize immediately the detection of initial earthquake generated seismic waves to effect immediate shut-off or control of certain types of equipment and apparatus (such as gas or gasoline lines and pumps, nuclear reactors, etc.) in order to prevent damage thereto or to the surrounding area. Such damage might occur were such equipment or apparatus to continue to operate during earth tremors or thereafter when the equipment or apparatus may have been damaged through quake activity.
There has been offered both in Japan and the United States by the Toyo Automation Company of Tokyo, Japan, a unit called VIBCOCK V-711 which includes a shock sensing element suspended by a ferrite magnet. When dropped by an earthquake, a stopper is moved to close a valve thereby shutting off a flow of gas. However, this is a very special unit which is not intended for use as a part of an accelerograph, or accelerograph system.
In earthquake disaster operations in municipalities it is sometimes difficult for those charged with setting up rescue and damage control activities to know where the greatest damage has occurred so that emergency equipment and personnel can be most effectively dispatched to such locations. Heretofore, the only way in which the desired damage intelligence has been collated is by telephone calls or two-way radio communication with the badly affected areas. However, where damage is great and telephone service has been interrupted, such information may be slow in getting out of such areas. An example of this was the Feb. 9, 1971 earthquake in the Los Angeles, California, area. Accelerographs heretofore devised may be suitable for recording what happened in the areas where they were located, but such historical information provides no assistance at the time the quake is occurring or immediately thereafter when emergency assistance is needed for the affected areas.